A hydraulic trip unit of the type previously referred to is the subject of DE 34 32 890 C2 which serves for the direct actuation of a fast-acting shut-off valve for a turbine plant, in which the monitoring of the hydraulic actuating pressure of the fast-acting shut-off valve is undertaken by three solenoid valves which are interconnected in a wire-free manner and interconnected in the style of a 2 out of 3 circuit. The known trip unit in this case trips as long as at least two of three solenoid valves change into a deenergized state, i.e. they revert to the respective neutral position. In the event of a trip, a portion of oil which flows along a control oil line which is connected directly to the fast-acting shut-off valve and in dependence upon the oil pressure which prevails along the control oil line enables an actuating element to operate, directly reaches the drain via a by-pass line which is provided by two solenoid valves, so that the control oil pressure along the control oil line abruptly reduces in the event of a trip as a result of which actuating means which are in the fast-acting shut-off valve are correspondingly activated for closing the said fast-acting shut-off valve. If at least two of the three solenoid valve units are energized, then no bypass passage from the control oil line to the drain is opened by the solenoid valves so that the control oil pressure along the control oil line remains uninfluenced. The known trip unit which is realized within the scope of a hydraulic block furthermore has manually operable or automatically working monitoring units for functional testing of the individual solenoid valves even during normal operation.
The advantage of the known trip unit is to be seen as that of the entire trip unit not being automatically activated in the case of there being a defect of a single solenoid valve as a result of which a turbine trip would occur, rather it being possible to operate the solenoid valves individually in sequence in a deenergized state in order to be able to test the correct function of the individual solenoid valves during turbine operation. In the case of the known trip unit, however, the only low throughflow capacity is disadvantageous. For modernizing hydraulic systems which are already in operation in prime mover plants it especially requires taking into consideration large volumetric flows at lower pressures which are to be controlled by the solenoid valves, which requires larger cross sections and throughflow capacities.
Furthermore, EP 0 540 963 B1 describes a feed circuit for a split hydraulic system which enables a pressurized fluid to be fed along a main line and which is provided for operating fast-acting shut-off valves and/or steam control valves which for example control the steam feed of a steam turbine. For feeding the pressurized fluid into the corresponding main line the known feed circuit makes provision for at least one sequence valve, which is formed as a slide valve, with which is associated a plate valve arrangement acting as a discharge booster. In the case of the known feed circuit, disturbing pressure shocks in the feed system can be largely excluded in all operating states, furthermore high throughflow capacities, as are necessary for use in steam turbines and gas turbines, can be achieved with this. On the other hand, the proposed feed circuit is complicated in its construction, however, and requires expensive components which not least bring about considerable costs.